In communications systems employing optical wave guides, such as optical fibers, two or more optical wave guides are typically required to link two communicating devices with one another. Thus, the optical wave guides must be coupled in series to one another so that where one optical wave guide ends another optical wave guide begins. Frequently, at such couplings between optical wave guides, the signal is also filtered to remove noise from the transmitted signal. In the case of optical fiber wave guides, before filtering, the optical signal emitted by the first optical fiber is typically spread and then collimated by a lens so that the signal can be received by the second optical fiber more easily without taking extraordinary measures to precisely align the ends of the fibers; graded index lenses, or so-called GRIN lenses, are generally used for this purpose. Filtering of the transmitted light is frequently accomplished by positioning an optical filter between the output end of the first fiber and the input end of the second optical fiber. The filter screens out unwanted wavelengths of light. In the case of narrow band optical noise filters, substantially all wavelengths except those within a narrow bandwidth of wavelengths, or passband, centered about a center wavelength are removed, thereby removing noise from the transmitted signal. The center wavelength of the filter is selected so that it closely matches the center wavelength of the signal emitted by the lens in the coupler, and the bandwidth of the filter is selected so that the filter will pass at least substantially all of the wavelengths of the signal, thereby eliminating most of the noise which is primarily transmitted at different wavelengths. The center wavelength of narrow band filters is dependent on the angle of the filter with respect to the incident light beam and, in the case of absorption filters, with the level of quality control maintained while the filters are manufactured. It is therefore important that the filter be placed and held securely in the signal path at a precise angle so that the passband selected matches the signal wavelength as closely as possible.
The optical characteristics of most optical filters, however, change with variations in temperature. Temperature changes can result from changes in the environment or heating of the filter caused by the optical signal, typically generated by a laser, impacting upon the filter. For example, when an optical filter is heated, the center wavelength of the passband typically increases. In the case of very narrow band optical filters, if the movement in the center wavelength is sufficiently large, the filter will filter out the entire signal, thereby only transmitting noise. In optical communications systems utilizing wide bandpass filters, the movement of the center wavelength may not be important, however, in newer long distance systems where one or more channels are required with narrow limits on transmitted bandwidths, the magnitude of temperature effects and the resolution required to match the filter to the transmitted signal becomes critical to the performance of the system. To ensure that heating does not eliminate the signal intended to be transmitted, a filter can be fabricated from material or in such a way to reduce the effect of temperature upon the passband. However, the process of manufacturing such filters is complex and inefficient in that many of the resulting articles must be discarded as defective. As a result, such filters are expensive. Another solution is to use low cost filters for which the center wavelengths are relatively susceptible to thermal variations but to maintain the filter at a constant temperature; however, maintaining a constant temperature is often impossible or uneconomical.
One may also, as another alternative, use filters that can be physically, electronically or magnetically altered to vary their filtering characteristics. Such active thermal compensating filters are highly complex, requiring thermal detectors and additional mechanisms to change the filter's optical characteristics as the temperature varies, all of which introduce and increase the possibility of malfunctions.
There is accordingly a need for a low cost, relatively simple optical fiber coupler employing inexpensive filters or other optical elements that can be easily and accurately tuned and that passively and reliably compensate for environmental and/or operational temperature variations.